Morphology and structure of the Aegean Sea

Deep-Sea Research, 1971, Vol. 18, pp. 109 to 122. Pergamon Pre~. Printed in Great Britain.

Morphology and structure of the Aegean Sea TERRY S.

MALEY*

and G.

LEONARD JOHNSON+

(Received 23 February 1970; in revised form 27 April 1970; accepted 28 April 1970)

Abstract--During the summer of 1968,a detailed bathymetric surVeyof the Aegean Sea was conducted by the U.S. Naval Oceanographic Office. The Aegean Sea has many morphological features typical of island arcs and some which are not typical. On the basis of geomorphological interpretation, the Aegean Sea is discussed in three parts: the southern; the central; and the northern Aegean Sea. The southern portion is interpreted as being part of a typical island arc structure. The central Aegean is occupied by an elevated aseismic plateau. The northern Aegean contains a series of steep-sided troughs and ridges. INTRODUCTION

A BATHYMETRICsurvey in the Aegean Sea was recently conducted by the U.S. Naval Oceanographic Office. This study combined with data from other surveys (WATSON and JOHNSON, 1969; GONCHAROVand MIKHAILOV, 1963), has resulted in a new bathymetric chart (Fig. 1). The physiography substantiates the hypotheses that the Aegean Sea is part o f an island arc system (RYAN, 1969 ; GmRMANN, 1966) which is however, typical in some respects and atypical in others. A number of investigators have published reports on the eastern Mediterranean (EMERY et al., 1966; RYAN, 1969; HERSEY, 1965; HARRISON, 1955; YEMEL 'YANOV et al., 1965; VOGT and Hicks, 1969; GONCHAROVand MIKI-L~ILOV,1963; MmI-IAILOV, 1965; GONCHAROV, 1961; WATSON and JOHNSON, 1969; BLANC and FROGET, 1967; OLAUSSON, 1961; PARKER, 1958). The land geology of the eastern Mediterranean has been thoroughly investigated, with perhaps the best general works including HOLMES (1965), KtrMMEL (1961) and AUBOUIN (1965). During this survey soundings were collected with an Ocean Sonics Recorder (OSR) precision recorder and the depths are accurate to within 1 in 3000 units. Depths were scaled at peaks and troughs and at 20-fm intervals. ~ A 25-fm isobath interval was used in contouring the sounding data. Data from various eharts and atlases were used to fill in areas of no data. The track lines were oriented east-west and spaced approximately five miles apart throughout the Aegean Sea with several north-south lines as cross checks. Navigation was by radar fixes o n the islands, Loran and satellite and is very accurate. EASTERN

MEDITERRANEAN

The eastern Mediterranean is dominated by a complex island arc structure. The Hellenic Are (GoNcHAROV and MIKHAILOV, 1963; EMERY et al., 1966; RYAN, 1969) extends from the mountain chains in Greece and, striking southeasterly, forms the *U.S.N. Oceanographic Office; present address, Geology Dept., University of Idaho. tU.S.N. Oceanographic Office. ~1 fathom = 1.8288m. Throughout this paper the abbreviation 'fro' is used for fathom. 109

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islands of Kithira, Crete, and Rhodes and thence passes into Turkey. Associated with the arc are relatively deep, parallel trenches and a broad outer arc, the Mediterranean Ridge (Fig. 2) (EMERY et aL, 1966; WATSON and JOHNSON, 1916). In crosssection the Hellenic Arc is similar to other island arc systems such as the Tonga Trench (FISHER and H~SS, 1963), in that it includes an outer ridge, fore deep, island arc, and within the Aegean Sea the inner deep and secondary volcanic islands (Fig. 2). Outer ridge and trench system

The Mediterranean Ridge (Fig. 2) to the south of the Cretan foredeeps is a zone of compression (RVAN, 1969; VOGT et aL, 1970). The region of extreme relief immediately south of Crete (Fig. 2) containing deep trenches and steep escarpments is the Hellenic Trough (EMERY et al., 1966). The topography curves gently as it strikes parallel to the Cretan Island Arc. The Strabo Trench borders the Mediterranean Ridge for 150 miles before diverging into a small basin southeast of Rhodes. The Pliny Trench lies just south of Crete with depths of 2240 fm (EMERY et aL, 1966; WATSON and JOHNSON, 1969). Cretan Arc

This arc is primarily emergent in the form of islands composed of folded Alpine sediments (Fig. 3). Sills between the islands are relatively shallow except between Rhodes and Scarpanto where the sill depth is between 500 and 600 fm (Fig. 1).

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Inner trough and secondary volcanic arc North of Crete a series of small elongate depressions with flat floors form the inner trough, here named the Cretan Trough the deepest part of the Aegean Sea (Figs. 4 and 5). The Cretan Trough is ' U ' shaped and parallels the Cretan Arc (Fig. 1). The basins become successively deeper towards the east with a depth of 1339 fm in the most easterly basin. Abyssal gaps connect some of these small basins (RYAN, 1969). Turbidite sequences (BLANC and FROGLrr, 1967) indicate that turbidity

114

TERRY S. MALEYand G. LEONARDJOHNSON

currents have been active in this region. There are 600 m of unconsolidated sediment above layer ' M ' (measured by seismic reflection methods) in the Cretan Trough (WATSON and JomqSON, 1969). The western arm of the Cretan Trough (Fig. 1, Fig. 6, profile 8 and Fig. 7, profile E), consists of two connected elongate basins; both basins have moderately flat bottoms and the deeper has a maximum depth of 702 fm. Across the northeastern branch (Fig. 7, profiles D and G) of the Cretan Trough (also Fig. 6, profile 8, extreme right) there appears to be step faulting on the walls (compare Fig. 7 profile E from the northwestern portion of the Cretan Trough). The deeper easterly portion of the trough lacks a fiat floor indicating that tectonic processes are dominant over sedimentary ones on the eastern extremity of the trench. The large number of earthquakes attest to active tectonism in the southern Aegean. Cyclades Arc

The Cyclades Arc, an inner volcanic arc (Fig. 4), is marked by volcanoes (Santorini and Nisyras) (Fig. 3), and a number of fumeroles on the islands of the arc (GEORGALAS,1962; NINKOVICHand HEEZEN,1965). This arc also forms the southern boundary of the central Aegean Plateau (Fig. 6, profiles 7 and 8). The arc lies parallel to the Cretan Trough in depths of generally less than 100 fm. The greatest depths are found on the northwestern limb where a maximum depth of F M Standia o

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Central Aegean Plateau The central Aegean Sea from 37 ° to 38 ° 30'N is basically aseismic. The southwestern portion of the Central Aegean Plateau is shallow with large areas shoaler than 100 fin (Figs. 1 and 6, profile 6). On the eastern edge of the central Aegean plate (Figs. 1 and 6, profile 5) there are three flat-floored basins; the northerly two of which are considerably deeper than the third--504 and 617 as opposed to 343 fln. The two deep basins called the Ikaria Basin are connected by the 400-fm isobath and together form another east-southwest trending elongate deep. This trough (Fig. 4), parallels the island arc formed by the islands lying between Andros, Tines, Mikonos, Ikaria and Samos. The lack of earthquakes indicates that there is no present tectonic activity in this region. These deep areas are flat-floored or at least sediment smoothed (Fig. 6, profile 5). This along with the collaborative evidence of no epicenters indicates

116

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that sedimentation is prevailing over tectonism. The northwest-southeast trend of this deep area and the elevated portion to the southwest, which contains the island of Ikaria, suggests it to be basically a horst-graben structure with some cross-faulting. Also on the central Aegean plate, there is evidence of both wide spread minor faulting (Fig. 8, profile B) and submerged drainage channels (Fig. 8, profile C). At 37 ° 30'N, 25 ° 40'E (Fig. 1) a narrow channel at a depth slightly greater than 150 fm separates the plateau from the Greek shelf to the west. RYAN (personal communication) has noted from seismic reflection data that this appears to be an erosional channel perhaps cut during a period of lower sea level. The sill at 37°N between 26 ° and 27°E at depths shoaler than 200 fm (Fig. 8, profile I) is the other major sill between the central and southern Aegean waters. N O R T H E R N AEGEAN SEA

Skiros Trough and Ridge The northern Aegean Sea contains a number of diverse, morphological features. At 38 ° 30'N (Fig. 4) the series of deeps can be connected to form a shallow arcuate trench system called the Skiros Trough. The western portion of the Skiros Trough is the deepest, in places exceeding 500 fm (Fig. 6, profiles 3 and 4). It consists of three

Morphology and structure of the Aegean Sea

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separate basins of 551, 529 and 527 fm m a x i m u m depth enclosed by the 300-fro isobath. The eastern portion of the Skiros Trough is shallower and less well defined with two basins of 263 and 363 fm (Fig. 1).

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A sinuous ridge (Skiros Ridge) located at 39°N and trending east-west (Fig. 6, profile 3) m a y possibly represent the inner are o f the Skiros Trough. Except for a narrow gap approximately five miles wide southwest of Skiros, this ridge averages 75 fm in depth and is not believed to exceed 100 fm at any other point as it winds across the central part of the Aegean Sea. The gap through Skiros Ridge provides a sill depth o f 175-200 fm between the water masses to the north and the central Aegean.

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Lesvos Basin Another elongate basin (Lesvos Basin) with a maximum depth of 537 fm lies about 39° 10'N and 25°E (Fig. 1). This deep has the same northeast-southwest orientation as the Anatolian Trough and is one of the three major basins of the northern Aegean Sea. Profile 3 (Fig. 6) indicates the relatively large size of this feature and the lack of a flat floor. This may be a portion of the Anatolian Fault Zone extending westward from Turkey (RYAN, personal communication).

Anatolian Trough Dwarfing all other features in the northern Aegean Sea is the northeast-southwest trending trough which extends across the entire sea (Figs. 1 and 6, profiles 1-2). Profile 1 (Fig. 6) crosses the graben at its greatest depth--826 fm on the eastern end. The second greatest depth is on the western end of the graben as shown on Fig. 6, profile 2. The graben broadens at its western extremity, giving it a wedge-shape as

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Morphology and structure of the Aegean Sea

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is apparent from comparison of (Fig. 6, profile 2), and K (Fig. 9) to profiles J, H and (Fig. 6, profile 1). The wails of the Anatolian Trough are precipitous (Fig. 6, profiles 1 and 2). The floor of the southwestern extremity of the graben arches gently and appears as overlapping hyperbolic echoes on (Fig. 9, profile K). Reflected echoes of this type may be produced by elongate ridges or scarps on an otherwise smooth sea floor. This and the earthquakes in the region are indicative of recent tectonism. Perhaps the tectonism is the result of tensional forces with resultant upwelling in the graben similar to those of the central rift of the Mid-Ocean Ridge. However, it should be noted that on allcrossings of this feature only profile K was characterized by hyperbolic echoes on the graben floor. In the graben zone (Fig. 9, profiles J and H) at its eastern extremity the walls appear to have been caused by a series of normal faults similar to those associated with the troughs in the southern Aegean. The northern Aegean Sea apparently is the result of several divergent trends. The remnants of the northeast-southwest land masses can be seen as isolated elevated banks often shoaler than 100 fm (Figs. 1 and 4). Almost perpendicular to these trends is the Anatolian Trough which perhaps is a large graben and apparently connects the eastern and western portions of the North Anatofian shear fault across the Aegean into the Trikkeri-Channel fault zone and thence southwest into the Gulf of Patras. The trough changes strike from west-southwest to southwest at a saddle of 275 fm at 25°E. SEISMICITY

Throughout the Mediterranean region, earthquake epicenters are generally confined to the Alpine orogenic belts. BARAZANGIand DOP.MAN(1969) have also reported epicenters for the Aegean and adjacent areas. In the southern Aegean Sea the earthquake activity lies parallel to and along the Hellenic Arc complex (Fig. 1). Deeper focus earthquakes generally occur beneath the Cretan Trough north of Crete. This is in conformity with the hypothesis of an inclined plane (BENIOFF, 1949) dipping beneath Crete from the Hellenic Trough (Fig. 4). The earthquake distribution further reveals a nearly aseismic plate in the central Aegean Sea (Fig. 1). SCHEIDEGGER(1964) and P~AZACrIOSand DELmASIS(1969) interpreted fault-plane solutions of earthquakes in the Hellenic Trough as resulting from stress perpendicular to the strike of the island arc. Based on focal mechanisms and plate boundaries, McKenzie suggested that rifting with resultant extension is occurring in the northern Aegean graben (Anatolia Trough) thrusting the central aseismic block southward over the Hellenic Trough (RYAN, 1969). A belt of intermediate depth earthquakes also exists in the northern Aegean Sea along the axis of the Anatolian Trough. BARAZANGIand DOgMnN (1969) suggest that the deeper epicenters circling the Aegean and forming a ' C ' with the open end toward Turkey are also associated with the rifting and block movements. It seems, however, that a reasonable alternative is to split them into two groups along an east-west line from central Turkey to the seismically quiet area of the Gulf of Patras in the Ionian Sea. MAGNETICS

A magnetic tranquil zone extends north from the Herodotus Abyssal Plain across the Cretan Arc to the volcanic arc. VOGTand HIGtS 0969) suggested that the material

120

TEV.RY S. MALEY and G. LEONARDJOHNSON

forming of ridge relief is probably composed of a non-magnetic sedimentary series. A secondary volcanic arc (Cyclades Arc) lies inside and parallel to the Hellenic Arc (Fig. 4). Locally this inner volcanic zone is characterized by high magnetic anomalies (C. MURPHY,personal communication). A negative gravity field lies over the Pliny and Strabo Trenches south of Crete (HARRISON,1955), which lends additional weight to the island arc concept for the Hellenic Arc (FISHERand HESS, 1963). SUMMARY

The sea floor morphology of the southern Aegean Sea and neighboring Levant Sea reveals a typical island arc physiography. The Hellenic Arc consists of an outer ridge (Mediterranean Ridge), deep fore trench (Pliny and Strabo Trenches), island arc (Cretan Arc), inner trench (Cretan Trough) and lastly a secondary volcanic-arc, the Cyclades Arc. Acting stress components of earthquakes within this area indicate that tensional forces parallel to the Hellenic Arc predominate. In Fig. 2, the physiography of the Hellenic Arc and Java Arc are compared. The similarities are: (1) Outer Ridge; (2) A deep fore-trench which in the Mediterranean is actually double; (3) The Cretan Arc is emergent; whereas the Bali Ridge lies at a greater depth. On other portions of the Java Arc the Bali Ridge does form several small islands; (4) The Bali Trough and Cretan Trough would seem to be similar features; (5) The Indonesia Arc is large and massive whereas the Cyclades Arc is a relatively small feature; (6) Both trenches are active seismically with well developed Benioff planes dipping beneath the island arc. The Hellenic Arc is considerably more complex to the north as an aseismic plate borders the trench complex which in turn is bordered by the ridges and troughs of the northern Aegean; whereas the Java Arc is bounded by a broad expanse of both emergent and submerged continental shelf. It seems reasonably certain that crustal downwarping of the African plate is occurring along a Benioff plane sloping under Crete. Therefore, it is likely that the Mediterranean Ridge is sea floor being intensely folded and elevated in response to crustal movements. Sediment cores (RYAN, 1969) suggest that the ridge is composed of normal deep sea sediments which are in the process of being lithified. The central Aegean Plateau is aseismic and is apparently acting as a plate (MOROAN, 1968) caught between the tectonic activity to the north and the northward moving African Plate to the south. Islands on this plate are folded Paleozoic and granitic masses (Fig. 3). The northern Aegean is the most geologically intriguing portion of the sea. Possible interpretations would include: (1) The Anatolian Trough represents an incipient rift valley. Magnetic anomalies do not support the theory that this deep is a center of sea floor spreading (C. MURPHY, personal communication). However, it could be argued that the rift is too young or spreading too slowly to have properly developed Raft-Mason lineations (VINE and MATrrmws, 1963). Profiles J and K (Fig. 9) could be interpreted as showing a topographic bulge in the floor of the rift indicative of axial growth. Likewise, profile H illustrates what appear to be small tensional faults. If extension is occurring along the graben, then the crust may be in the process of being carried beneath the Central Aegean Plate which lies 100 km to the south,

Morphology and structure of the Aegean Sea

121

with the Skiros Trough acting as a proto-trench. The system presumably has not existed long enough to have created a Benioff plane beneath the central aseismic plate. Due to its proximity, the continental mass north of the graben may be causing a steep dipping plane and thereby creating the intermediate focus earthquakes over 100 km deep just north of the Anatolian Trough. (2) The Anatolian Trough may merely represent the eastern extension of the Anatolian transcurrent fault. This great fault (1600 km) is dextral with a total inferred displacement of 350 km (WELLMAN, 1969). (3) A morphological case can be made for considering the entire northern Aegean Sea as a developing trench system (Fig. 4). The narrow deep Skiros Trough would be the fore trench. The parallel islands and shoals (Skiros Ridge) would act as a developing island arc with the Anatolian Trough the inner trench. This would imply that the North Aegean graben is acting as an inner trench similar to the Cretan Trough. However, the foredeep is classically the greater of the two trenches which tends to negate the concept o f an incipient island arc system developing in the northern Aegean Sea. Also the inner volcanic arc is missing. An island arc system would explain the few earthquakes of intermediate depth in the northern Aegean if it is assumed that crustal downwarping is occurring along a Benioff plane. However, the Skiros Trough seems seismically too tranquil to permit this hypothesis. (4) It is possible that the Central Aegean plate, in response to pressure from the south, is moving slightly northward with resultant crumpling of the sea floor and perhaps even some crustal downwarping in the zone of intermediate earthquakes. In conclusion, it is difficult to say which of the above is most nearly correct. The complex tectonic patterns may very well be a combination or some other process not divined by the authors. Acknowledgements--The authors wish to acknowledgethe assistance of the U.S. Naval Oceanographic Office personnel who participated in this survey. The paper was reviewed by W. F. B. RYANof Larnont-Doherty Geological Observatory, who offered many helpful suggestions which were incorporated in the text. REFERENCES ANONVMOOS(1962) Carte Tectonique Internationale de l'Europe, Congress G~ologique Internationale sous commission de la Carte Tectonique du Monde. AtrnOtnN J. (1965) Developments in Geoteetonics, Part I: Geosynclines, Elsevier, New York, 335 pp. BARAZANGI M. and J. Dol~t~N (1969) World Seismicity Maps, compiled from ESSA, Coast and Geodetic Survey, Epicenter Data, 1961-1967. Bull. Seismol. Soc. Am., 59, 369-380. BENIO~ H. (1949) The fault origin of oceanic deeps. Bull. geol. Soc. Am., 60, 1837-1866. BLANCJ. and C. FROG~T(1967) Campagne de la Calypso en Mediterranee Orientale. Annls Inst. Ocdanogr. Monaco, 45, 257-291. EMERYK. O., B. C. HEEZeNand T. D. ALLAN(1966) Bathymetry of the Eastern Mediterranean Sea. Deep-Sea Res., 13, 173-192. FISHER R. L.-(1964) A Preliminary Report on Expeditions Monsoon and Lusiad. Ref Scripps Inst. Oceanogr., 64-19, 237 pp. (Unpublished manuscrip0. FlsrmR R. L. and H. H. HEss (1963) Trenches. In: The Sea, M. N. HILL, editor, Interscience, New York, 3, 411--436. GEOROALASG. C. (1962) Catalogue of the active volcanoes and solfatara fields of Greece. In: Catalogue of the Active Volcanoes of the World including Solfatara Fields, Int. Ass. Volcanol., Rome, 7-10. GmRMANNG. (1966) Gedankan zwei Ostmediterranen Sehwelle, Bull. Inst. Ocdanogr. Monaco, 669 16 pp.

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TERRYS. MALEYand G. LEONARDJOHNSON

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